Certain derivatives of purine or pyrimidine bases have shown antiviral and antitumor activity. For example, see EP 0 173,624; EP 0 253,412; EP 0 353,955; WO 92/01698; EP 0 481,214; and J. Org. Chem. 57: 2320-2327 (1992). The compounds of the present invention represent novel compounds derived from purine and pyrimidine bases.